Hydroxy-terminated polyether binders for composite rocket propellants
Propellants based on cross-linked Hydroxy Terminated PolyEther (HTPE) binders are being used as alternatives to Hydroxy Terminated PolyButadiene (HTPB) compositions. HTPE propellants have similar mechanical properties to HTPB propellants but they give a less severe response in ‘slow cook-off’ tests for IM compliance. A literature review is presented on the development and properties of HTPE propellants in an attempt to place them in relation to recent trends in Insensitive Munitions. To gain a better understanding of the behaviour of HTPE propellants an HTPE pre-polymer and a range of binder network samples with different NCO/OH equivalence ratios, with and without plasticizer, have been synthesised and characterised by a range of techniques. The thermal decomposition of the HTPE binder network and propellant samples were also studied. Desmodur N-3200 was used as a curing agent and n-BuNENA as an energetic plasticizer. Similar analyses were performed on analogous HTPB pre-polymer and binder network samples and the results were compared with those obtained for the corresponding HTPE samples. Two kinds of HTPE propellant were manufactured containing HTPE pre-polymer, n-BuNENA, 2NDPA and either AP or AP+PSAN as oxidiser. Also HTPB propellant was prepared. Small cook-off test vehicles (SCTV) were filled with HTPE and HTPB propellants and slow cook-off tests were performed. In contrast to HTPB binders, which become harder during slow heating, it was found that the HTPE binders soften under the same conditions. This behaviour is possibly due to chain scission of the soft and hard segments of the HTPE polymer matrix. Thermooxidative processes and reactions of the energetic plasticizer decomposition products are believed to be the responsible for the scission of the polymeric matrix. From the binder characterisation and slow cook-off results it is concluded that there is a relation between the degree of polymeric matrix scission during slow heating and the violence of the response at the point of self ignition. This underlies the main difference between HTPB and HTPE propellants in slow cook-off. While HTPB compositions become harder and more brittle, HTPE propellants become softer and have a lower surface area at the self ignition point.